Directional coupler having plural slanted identical coupling slots of critical length



March 1, 1966 E. SALZBERG 3,238,473

DIRECTIONAL COUPLER HAVING PLURAL SLANTED IDENTICAL COUPLING SLOTS OF CRITICAL LENGTH Filed Nov. 28, 1961 2 Sheets-Sheet 1 I 12% A I 4 c I/ I/ 2 B F (PRIoR ART) D A-- 0 d) C). 0

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26.0 I I I I I I FREQUENCY KMC) F I G 5 INVENTOR. EDWARD SALZBERG ATTORNEYS 'March 1, 1966 E. SALZBERG DIRECTIONAL COUPLER HAVING PLURAL SLANTED IDENTIGAL COUPLING SLOTS OF CRITICAL LENGTH 2 Sheets-Sheet 2 Filed NOV. 28, 1961 11L Fl G74 FIGS FIGG

INVENTOR.

EDWARD SALZBERG WW ,mwm

ATTORNEYS United States Patent DIRECTIONAL COUPLER HAVING PLURAL SLANTED IDENTICAL COUPLING SLQTS 0F CRITICAL LENGTH Edward Salzberg, Framingham, Mass., assignor to Microwave Development Laboratories, Inc., Wellesley, Mass, a corporation of Massachusetts Filed Nov. 28, 1961, Ser. No. 155,265 3 Claims. (Cl. 333-10) This invention relates in general to the coupling of a portion of the electromagnetic wave energy in one waveguide to a second waveguide in a manner such that the energy coupled into the second waveguide is constrained to travel in one direction only. More particularly, the invention pertains to a directional coupler of the type em ploying rectangular waveguides having a common narrow wall in which apertures are provided to permit energy in one waveguide to couple into the other waveguide.

The invention resides in an arrangement of coupling slots whose configuration and orientation permit the degree of coupling, that is, the amount of energy transferred from one waveguide to the other, and the degree of directivity of the directional coupler to be closely controlled. A significant feature of the invention is that the degree of coupling is maintained relatively constant over a large band of frequencies.

; In accordance with the invention, a series of elongated narrow slots are cut into the common narrow wall of a rectangular waveguide coupler so that energy from the main guide couples through the slots into the auxiliary guide. Each slot is arranged so that its longitudinal axis is inclined at an angle 0 with respect to a line, passing through the slots center. The number of slots employed in the coupler is determined, essentially, by the amount of energy which is to be transferred between the branches of the coupler. The centers of the slots are spaced, preferably, at quarter wave length intervals. In order to control the directivity of the directional coupler, the slots are inclined so that the angle 0 decreases at each end of the series of slots. Since the angle of tilt, 6', has a marked effect on the amount of energy coupled through the slot, the number of slots in the series is dependent also upon the degree of directivity which the coupler is required to have.

The invention, both as to its construction and mode of operation, can be better apprehended from a perusal of the following exposition which is to be considered in conjunction with the accompanying drawings in which:

FIG. 1 depicts a directional coupler of conventional design;

FIG. 2 shows a conventional directional coupler employing four holes in its common side wall;

FIG. 3 graphically illustrates the coupling characteristics of a device constructed according to the invention contrasted with the coupling characteristics of a conventional coupler;

FIG. 4 depicts the common wall of a Waveguide directional coupler constructed in accordance with the invention;

FIG. 5 is an orthographic view of a coupler employing the slotted common wall of FIG. 4; and

FIG. 6 depicts the common wall of another embodiment of the invention.

Referring now to FIG. 1, there is shown a directional coupler of conventional design which can be used, for example, to monitor a waveguide line for reflected Wave energy. Two waveguides 1 and 2 are connected by coupling slots 3 and 4, usually circular holes, in their common narrow wall 5, Some of the wave energy traveling in the main waveguide from port A toward port C couples through slots 3 and 4 into waveguide 2. Assuming that 3,238,473 Patented Mar. 1, 1966 ports B, C, and D are terminated in a manner preventing wave energy reflections, when coupling slots 3 and 4 are spaced by one quarter of the wavelength of the energy in the guide the phases of the energy in the guides are such that cancellation of energy occurs in the direction of port B whereas the energy travelling toward port D is additive. Provided the apertures 3 and 4 are of the appropriate size, complete cancellation of energy in one direction can be obtained so that all the energy in guide 2 proceeds toward port D. The conventional directional coupler is symmetrical in operation, that is, energy of the appropriate frequency introduced at port C into guide 1 appears at ports A and B but none appears at port D. The apertures 3 and 4 are relatively small openings, where it is not desired to couple more than a small fraction of the energy from the main guide 1 into the auxiliary guide 2, as, for example, when the power in the auxiliary guide 2 is thirty decibels (30 db) or more below the power in the main guide.

Where a large amount of power is to be coupled from the main guide into the auxiliary guide, the apertures 3 and 4 cannot be greatly enlarged to permit more energy to be transferred since the hole size is limited by interference caused by the ICC & 4

spacing and by the height of the waveguide. To overcome that difiiculty, it is common to use three or more smaller holes which are spaced at quarter wave intervals. For example, FIG. 2 shows a conventional design ernploying four circular holes in the common Wall between the main and auxiliary guides. The four holes are spaced at intervals equal to one quarter of the mean wave length of the energy propagating in the guide. The end coupling holes are reduced in diameter to facilitate energy cancellation in the reverse direction over a broad band of frequencies. The four hole coupler is somewhat frequency sensitive with respect to its directivity since complete cancellation of energy in one direction does not occur when the coupler is operated at frequencies where the holes are spaced at substantially half wave lengths or multiples of half wave lengths. The frequency sensitivity of such couplers with respect to their directivity can be reduced by adding more holes. A four hole coupl er, therefore, is less frequency sensitive than a two or three hole coupler.

With regard to directivity, the two hole coupler of FIG. 1 is similarly frequency sensitive and to a larger extent than the four hole coupler. In general, the frequency band accommodated by a directional coupler for a given minimum directivity can be extended by increasing the number of coupling apertures.

When a directional coupler is used over a band of frequencies, the degree of energy coupling in a forward direction between the guides varies as the operating frequency is changed. In FIG. 3, the curve 6 shows the amount of energy coupled through the four hole coupler of FIG. 2 as a function of the frequency of operation. It is apparent that as the frequency of operation is increased, the amount of coupling is noticeably decreased. Between 8.2 kmc. and 12.4 kmc., the coupling of the multi-hole device changes from 27 db to .33 db, a variation of 6 db.

Curve 7 of FIG. 3 depicts the coupling characteristic of a five slot coupler embodying the invention. That five slot device is shown in FIG. 4. It is evident in FIG. 3 that curve 7 is symmetrical about the mid-band frequency.

Between 8.2 kmc. and 12.4 kmc., the coupling of the fi ve slot device changes from 28.2 db to 30.2 db, a variation of 2 db.

In the directional coupler of FIG. 4, the slots are substantially rectangles with rounded off corners. Slots 11 and 15 form an angle 6 with a vertical reference line. Slots 12 and 14 are inclined at a more acute angle 6 with respect to the vertical. Slot 13 is set with respect to the vertical at an angle 6 which is larger than 6 By way of example, 6 may be 10, 6 may be 30, and 6 may be 40. The slots are cut about centers which are spaced at quarter wave length intervals.

An infinitely thin slot in the narrow Wall of the waveguide, which slot is perpendicular to the flow of wave energy will, theoretically, not couple energy. However, as the slot is tilted from the perpendicular, the angle of tilt being 6, the voltage coupling increases in direct relation to the increase in sin 6, assuming the dimensions of the slot remain the same for all angles of tilt. Thus, where V is the voltage coupled into the auxiliary guide when the slot is at an angle 6 with respect to the perpendicular, and V is the coupled voltage when the slot is at an angle 6 then K g Sln 9g V1 SlD 61 It has been found that if the angle of tilt, 6, is fixed and the slot length is less than the change in coupled voltage will vary approximately as the cube of the length ratios. That is, if V is the voltage coupled into the auxiliary guide through a slot length l and V is the voltage coupled through the slot when its length is increased to 1 then assuming, of course, that 6 is constant. As the slot length approaches x -+2, the voltage coupling ratio increases in a complex fashion the cube of the slot length ratios. A point is therefore reached where the coupling at P (FIG. 3) equals the coupling at F The resulting slot length is generally between A; and V of the wave length at P depending somewhat on the thickness of the slot, but is reasonably independent of the angle 6. This is now the optimum length. The coupling of each slot can now be changed simply by changing the angle 6. The degree of coupling of each slot is adjusted in this manner to get the overall coupling and directivity characteristics as is done with the narrow wall multi-hole coupler. However, the flatness of coupling with frequency is maintained to approximately 2 db over a 1.5 frequency ratio as compared to 6 db over a similar frequency ratio when using round holes. It is to be understood that A refers to the wave length in free space.

It is evident that in the five slot coupler of FIG. 4, most of the energy passes through slots 12, 13, and 14 so that the amount of coupling provided by slots 11 and 15 is relatively small. The end slots, however, are required to give the coupler the desired directionality; that is, the end slots are effective, over the couplers frequency band, in causing energy cancellation in one direction in the auxiliary guide. The taper, i.e., the change in the angle 6 as one proceeds outwardly from the center of the coupler, which determines the angles of the end slots is usually designed to give a T chebychefl distribution. For a discussion of Tchebycheff distribution, see US. Patent No. 2,871,452 and the item entitled Super Directivity with Directional Coupler Arrays by H. J. Riblet in the Proceedings of the IRE, vol. 40, No. 8, August, 1952.

The amount of coupling can be increased by placing vmore coupling slots in the side wall,- For example, a

db coupler was constructed of rectangular waveguide such as shown in FIG. 5 having twenty sidewall slots. The common wall of .such a coupler is shown in FIG. 6 where an intermediate portion of the coupler is omitted, so that only the first five slots at the ends appear. All the slots are identical in width and length and are spaced on centers at quarter wavelength intervals. The end slots 21 and 31 form an angle with respect to the vertical (angle 6) of 8, slots 22 and 32 have an angle 6 of 23, and slots 23 and 33 have an angle 6 of 33. All the other slots, that is, the slots 24, 34- and the slots therebetween, are set at an angle of 43 with respect to the vertical. The slots are about one quarter of a wave length long and about one sixteenth of a wavelength in width. The effect on the amount of coupling caused by changing the length of a slot has previously been explained. The wave length referred to above, it should be understood, is the wave length of the energy in the guide oscillating at the midband frequency of the coupler.

Regarding the coupler of FIG. 6, it is apparent that the desired amount of coupling (10 db) could have been obtained with less than twenty sidewall slots by increasing the angle 6 of the center slots. However, the amount of coupling per slot has been purposely decreased in the coupler of FIG. 6 so that more slots could be used in order to obtain high directivity over a broad band width.

By employing the teaching of this invention, it is feasible to construct a directional coupler having a high degree of directivity over a large band of frequencies while maintaining the degree of coupling more constant over that frequency band. It is possible, by suitable design, to choose among a wide range of degrees of coupling while retaining adequate directionality.

Since changes, obviously, may be made in the embodiments of the invention depicted in the drawings, it is intended that those embodiments shall be deemed to be only exemplars of what may be achieved by the inventive concept here disclosed.

I claim:

1. A directional coupler of the type employing rectangular waveguides having a common wall in which a series of apertures are provided permitting energy to couple from one waveguide to another, the apertures being elongate slots of identical configuration, the geometric centers of the slots being spaced at regular intervals along the length of the common wall, the longitudinal axis of each slot being inclined at an angle with respect to a line passing through the slots center normal to the broad walls of the waveguides, the length of each slot being between and of the wavelength at P and the terminal slots of the series being less inclined than the other slots of the series.

2. A directional coupler of the type employing rectangular waveguides having a common narrow wall in which a series of apertures are provided permitting wave energy to couple therethrough, the apertures being thin elongate slots of identical configuration, the geometric centers of the slots being spaced at quarter wave length intervals along the length of the common wall, the longitudinal axis of each slot being inclined at an angle with respect to a line passing through the slots center normal to the broad walls of the waveguides, the length of each slot being between and of the wavelength at F and the slots being arranged at decreased angles of inclination at each end of the series of apertures.

3. A directional coupler of the type employing rectangular waveguides having a common narrow side wall in which a series of apertures are provided permitting wave energy to pass therethrough, the geometric centers of the apertures being spaced at quarter wavelength intervals, the coupler being characterized in that the apertures are thin elongated slots of identical configuration, the longitudinal axis of each slot being inclined at an angle with respect to a line passing through the slots center normal to the broad Walls of the waveguides, the length 5 6 of each slot being between and A of the wavelength 2,932,823 4/1960 Beck et a1 343-771 at F a plurality of the intermediate slots in the series 2,976,499 3/1961 Sferrazza 33310 having the same inclination, and the slots outward of said FOREIGN PATENTS intermediate slots being arranged at progressively decreased angles of inclination. 5 604,912 9/1960 Canada OTHER REFERENCES References Cited by the Examiner Hensperger: The Design of Multi-Hole Coupling Ar- UNITED STATES PATENTS rays, Microwave Journal, August 1959, pages 38 to 42. 2,573,746 11/1951 Watson et a1 343-771 2,676,257 4/1954 Hebenstreit 343 771 10 ELI LIEBERMAN, Acting Primary Examiner. 2,820,203 1/ 1958 Sferrazza 33310 HERMAN KARL SAALBACH, Examiner. 

1. A DIRECTIONAL COUPLER OF THE TYPE EMPLOYING RECTANGULAR WAVEGUIDES HAVING A COMMON WALL IN WHICH A SERIES OF APERTURES ARE PROVIDED PERMITTING ENERGY TO COUPLE FROM ONE WAVEGUIDE TO ANOTHER, THE APERTURES BEING ELONGATED SLOTS OF IDENTICAL CONFIGURATION, THE GEOMETRIC CENTERS OF THE SLOTS BEING SPACED AT REGULAR INTERVALS ALONG THE LENGTH OF THE COMMON WALL, LONGITUDINAL AXIS OF EACH SLOT BEING INCLINED AT AN ANGLE WITH RESPECT TO A LINE PASSING THE SLOT''S CENTER NORMAL TO THE BOARD WALLS OF THE WAVEGUIDES, THE LENGTH OF EACH SLOT BEING BETWEEN 3/8 AND 7/16 OF THE WAVELENGTH AT FH, AND THE TERMINAL SLOTS OF THE SERIES BEING LESS INCLINED THAN THE OTHER SLOTS OF THE SERIES. 